A control method of this kind and a control arrangement of this kind are disclosed in German patent publication 4,220,247. There, a first microprocessor or computer is provided to control the drive unit. In the preferred embodiment, the microprocessor or computer executes computations for metering fuel, adjusting ignition time point and for controlling the air supply to the engine in dependence upon the command of the driver. Most importantly, this computing element has the possibility to increase the power of the engine by computing the air supply. For this reason, a second computing element is provided to monitor the correct function of this computing element. The second computing element monitors the operation of the first computing element and, in the case of a fault, switches off the power control by the first computing element or initiates an emergency operation. In the preferred embodiment, the monitoring of the operation of the first computing element is performed by a comparison of the driver command to the position of the element influencing the air supply.
Even though this known system guarantees a reliable operation of a drive unit with high availability, it is associated with high complexity and therefore high cost because of the second computing element so that this solution can be unsatisfactory for other embodiments.